This invention relates to an image processing device for processing an image on an original document read with use of a scanner, for example, and an image forming device having the image processing device.
In recent years, a hard copying machine capable of multilevel recording by controlling the size or the density of dots in units of pixel has been widely known. The multilevel recording is advantageous in that highly precise tone expression can be attained in comparing with the pseudo-tone expression by the binary recording, and the texture can be represented smoothly. On the other hand, it is difficult to record each level of the multilevel recording constantly at a stable state. Particularly, the recording of highlighted portions (i.e., small dots or dots having a low density) on a sheet is very difficult, and inconsistencies in density may occur in the image.
In order to solve the problem, there has been proposed a method (Japanese Patent KOKAI Application No. 8-125863) which has been known as a prior art by Oda et al. This method is intended to prevent the fluctuation of the recording level of the highlighted portions in the image. According to this method, a plurality of tone conversion tables are prepared, and the level of the highlighted portions is converted with use of one of the tone conversion tables to a zero-level at which no tone is applied on the sheet. With use of the other tone conversion tables, the levels of the highlighted portions are stabilized by converting the levels of the high-lighted portions to a higher levels at which tone is stably applied to the sheet. By periodically switching the plurality of tone conversion tables for each pixel, the pixels in the highlighted portions are set to have a stable large size or a high density, otherwise, set to form no dot. The pixel forming no dot is also stable in a point of view.
This technique, however, employs a plurality of tone conversion methods are independent from each other, and the density of each pixel is converted irrelevant to the values of the pixels surrounding the pixel to be converted. Accordingly, this method is effective for the image having a smooth density distribution, but is ineffective for the image pattern or character in which density changes bit by bit. In such an image, the original image density is not maintained after the conversion, particularly, in the image having a pattern frequency approximate to the tone conversion switching frequency, moirxc3xa9 patterns will be generated due to the conversion.
According to this technique, the pixel the level of which is to be converted into the zero-level at which no tone is applied is determined by the positions of the pixel. In this case, non-zero level pixels are present only on the determined pixel, in the other portion, the density of the entire image is lowered.
Further, the tone conversion switching frequency corresponds to the tone conversion table one to one. Accordingly, in order to record the image having a plurality of tone conversion switching frequency, the corresponding number of tone conversion tables to that of the switching frequencies need to be provided resulting in the increase of the circuit size.
The image having a plurality of tone conversion switching frequencies can be converted with use of one tone conversion table if the contents of the tone conversion table is rewritten one by one in accordance with a plurality of tone conversion switching frequency. However, it is difficult to switch the frequencies in the same image due to the limit in the rewriting speed.
As described above, according to the technique of preventing the problem due to the highlighted portions by periodically switching a plurality of independent tone conversion methods, the image having high frequency components may be deteriorated, or the periodic pattern cannot be determined.
The object of the present invention is thus to provide an image processing device which can maintain original image density, prevent the generation of moirxc3xa9 patterns and deterioration of the image, and reduce the frequency of the pattern of highlighted portions to form a pattern having a flexible period, and an image forming device having the image processing device.
According to the present invention, the above-mentioned object is attained by an image processing device comprising:
input means for inputting a plurality of pixel data which constitute an image;
determination means for determining whether or not a level of a value of interested pixel data in the pixel data input by the input means is lower than a specific pixel value level;
adding means for, when the determination means determine that the level of interested pixel data is lower than the specific pixel value level, shifting the interested pixel data from a position at which the interested pixel data is input, to a position of the other pixel data, and adding the interested pixel data to the other pixel data; and
output means for outputting the pixel data processed by the adding means.
According to the present invention, the above-mentioned object is also attained by an image processing device comprising:
line structure forming means for forming a line structure;
dot structure forming means for converting the line structure formed by the line structure forming means to a dot structure in accordance with image density; and
image processing means for converting an input first image signal into a second image signal with use of the line structure forming means and the dot structure forming means, in which a high density portion is converted into a line texture, and a low density portion is converted into a dot texture.
According to the present invention, the above-mentioned object is also attained by an image processing device comprising:
input means for inputting a plurality of pixel data which constitute an image;
determination means for determining whether or not a level of a value of interested pixel data in the pixel data input by the input means is lower than a specific pixel value level;
computing means for, when the determination means determine that the pixel value level of interested pixel data is lower than the specific pixel value level, computing a shifting amount by which the interested pixel data is shifted from a position at which the interested pixel data is input, to a position of the other pixel data;
calculation means for calculating a pixel position signal of a pixel to be shifted on the basis of a pixel clock signal of the pixel data in a main scanning direction and a line clock signal of the pixel data in a sub scanning direction;
adding means for shifting the interested pixel data by the shifting amount from a position at which the interested pixel data is input to a position of the other pixel data, which is computed by the computing means on the basis of the pixel position signal output from the calculation means, the adding means adding the shifted interested pixel data to the other pixel data; and
output means for outputting the pixel data processed by the adding means.
According to the present invention, the above-mentioned object is also attained by an image processing device comprising:
input means for inputting a plurality of pixel data which constitute an image;
determination means for determining whether or not a level of a value of interested pixel data in the pixel data input by the input means is lower than a specific pixel value level;
adding means for, when the determination means determine that the level of interested pixel data is lower than the specific pixel value level, shifting the interested pixel data from a position at which the interested pixel data is input, to a position of the other pixel data, and adding the interested pixel data to the other pixel data;
output means for outputting the pixel data processed by the adding means; and
image forming means for forming the image on the basis of the pixel data output from the output means.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.